Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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26.2 Block Copolymer Gels 809

scale ﬁ llers on morphology and property development in block copolymer gels.

Electron microscopy images of gels modiﬁ ed with colloidal silica nanoparticles

and a surface - modiﬁ ed organoclay are provided in Figures 26.14 and 26.15 , and

indicate the extent to which the additives disperse, which is of paramount impor-

tance with regard to controllable property development. Mechanical properties are

generally found to improve when surface - modiﬁ ed silicas [49, 78] and organoclays

[78] are incorporated into the gel matrix. Efforts to use carbon nanotube s ( CNT s)

[78] have yielded less impressive results due to challenges associated with sufﬁ -

cient dispersion. An enhanced modulus is achieved when the additive is more

Figure 26.14 Energy - ﬁ ltered TEM image of a

triblock copolymer organogel composed of

10 wt% SEBS copolymer and modiﬁ ed with

3 wt% colloidal silica nanoparticles. The

styrenic micelles, stained by the vapor of

RuO

(aq), appear light, whereas the siliceous

nanoparticles appear bright, due to imaging

at an energy loss of 200 eV. Two neighboring

individual nanoparticles are circled.

Figure 26.15 Energy - ﬁ ltered TEM image of the same triblock

copolymer organogel pictured in Figure 26.14 , but modiﬁ ed

with 3 wt% organoclay. The siliceous clay platelets likewise

appear bright due to imaging under the same conditions as

those employed in the previous ﬁ gure.

810 26 Nanostructured Organogels via Molecular Self-Assembly

compatible with the matrix (solvent and midblock), as this renders improved dis-

persion and, consequently, more efﬁ cient stress absorption from the soft phase

[49] . Properties are observed [79] to generally improve with increasing nanoﬁ ller

content, even at surprisingly high (60 wt%) loading levels. The addition of nano-

particles may also expand the mechanical performance and stability of block

copolymer organogels at high temperatures, especially if (i) the attractive interac-

tions between the nanoparticles and matrix are particularly strong [49] ; or (ii) the

nanoparticles themselves form a secondary, load - bearing network that remains

thermally stable [78] .

26.2.5.2 Polymeric Modiﬁ ers

Endblock - Selective Homopolymer

In the previous section, the addition of inor-

ganic nanoﬁ llers to a block copolymer gel resulted in a hybrid material wherein

the nanoﬁ llers were highly dispersed to yield nearly discrete nanoscale particulates

with an ultrahigh surface - to - volume ratio. For this reason, and to avoid macro-

scopic phase separation between the nanoﬁ llers and the gel, only very low nano-

ﬁ ller concentrations can be considered. The addition of an endblock - selective

homopolymer to a block copolymer gel can likewise result in several different

scenarios, depending on factors such as endblock compatibility, molecular weight

disparity, and homopolymer concentration [84] . If the homopolymer is chemically

identical (hA) to the endblocks of an ABA triblock copolymer, then only the

molecular weight disparity ( α = N

/ N

) and hA concentration constitute key

design parameters. As in solvent - free block copolymers, if α is large ( > 1), the

hA molecules will not be physically accommodated within the brush comprising

the A - rich microdomains. In this case, the brush is said to remain dry due

to the lack of penetration of homopolymer molecules [85] . This entropic penalty

favors macrophase separation between the copolymer and homopolymer

molecules even at relatively low hA concentrations. In this limit, hA - rich domains

measuring on the order of micrometers or larger coexist with the gel network,

and the accompanying mechanical properties are largely dictated by the

separating interface. The incorporation of semicrystalline syndiotactic polystyrene

( sPS ) into a SEBS gel, for example, results in the formation of discrete sPS

crystals, which appear as ﬁ laments and sheets (cf. Figure 26.16 ) that greatly

improve the modulus due to adhesion between the crystals and the styrenic

micelles [86] .

As α becomes smaller, however, due to a reduction in N

or an increase in N

the smaller hA molecules can locate within the A - rich microdomains and wet the

compatible block brush. In this limit, added hA can serve to facilitate, or even

induce, copolymer micellization because of the corresponding increase in the

population of unfavorable A – B contacts [87] , and it can likewise promote a change

in interfacial curvature and, hence, gel morphology [88, 89] . Mechanical properties

are found [90] to generally improve with increasing hA fraction up to a molecular -

weight - dependent level, beyond which macrophase separation occurs. One way to

lessen the propensity for macrophase separation and to ensure the encapsulation

of a homopolymer within the endblock - rich microdomains responsible for stabiliz-

26.2 Block Copolymer Gels 811

ing the gel network is to increase the homopolymer/endblock compatibility. In

the case of gels composed of styrenic triblock copolymers (i.e., copolymers with

PS endblocks), poly(2,6 - dimethyl - 1,4 - phenylene oxide) ( PPO ) constitutes an ideal

candidate in this regard, since χ between these two polymers is negative over all

compositions and a large temperature range [91] . Moreover, since PPO possesses

a relatively high T

( ∼ 210 ° C), it may be added to improve the service temperature

of styrenic SAMINs [92] . This increase has been reported [93] to be as high as

∼ 30 ° C upon the addition of 3 wt% PPO to a SEBS gel. At higher loading levels,

the mechanical properties improve substantially and morphological transitions

can be expected [91] .

Cosurfactant Although a variety of midblock - selective homopolymers can be

blended into block copolymer gels (e.g., polyoleﬁ ns such as polypropylene [94]

added to gels with a primarily aliphatic solvent) to modify process or application

properties, such modiﬁ cation normally results in the formation of macrophase -

separated systems consisting of homopolymer - rich and gel - rich domains that are

discrete or cocontinuous, depending on the relative concentrations. For this

reason, such multicomponent systems are not considered further here. Another

means by which to alter gel properties at the molecular level involves the addition

of an AB diblock copolymer as a cosurfactant to the ABA triblock copolymer

network. In this scenario, the AB molecules, if sufﬁ ciently incompatible, are forced

to coreside with their ABA analogues, resulting in submicrodomain stratiﬁ cation

[95] . Due to the presence of AB molecules, the ABA molecules are entropically

forced to form bridges (rather than re - enter to form loops) due to coronal volume

exclusion, in which case addition of an AB copolymer in small quantities can

improve mechanical properties even when c < cgc for the parent ABA solution (cf.

Figure 26.16 TEM image of a triblock

copolymer organogel composed of 8.5 wt%

SEBS copolymer and 1.5 wt% syndiotactic

polystyrene ( sPS ). The styrenic micelles and

crystalline sPS sheets and ﬁ laments are both

selectively stained and appear dark. Adapted

from Ref. [86] and used with permission from

the American Chemical Society.

812 26 Nanostructured Organogels via Molecular Self-Assembly

Figure 26.17 ) [96, 97] . Complementary SAXS studies performed by Vega et al . [29]

reveal that the presence of an AB copolymer could help to prevent macrophase

separation due to a reduction in intermicellar distance, and could, in general, be

used to tune to the phase behavior of the system (cf. Figure 26.18 ).

26.2.6

Nonequilibrium Mesogels

Thermoplastic elastomer gels are normally prepared by mixing a triblock copoly-

mer and a low - volatility solvent, with or without a carrier solvent (to reduce viscos-

Figure 26.17 Dependence of G ′ on the

fraction of AB diblock copolymer added to

triblock copolymer organogels and solutions

varying in total copolymer concentration (in

wt%): 15 (

䊊

), 11 (

䊉

), and 7 (

䉭

). Note that a

frequency - independent modulus indicative of

a gel network is not achieved in the system

with 7 wt% copolymer until the diblock

copolymer is added. The solid lines serve to

connect the data. Adapted from Ref. [96] and

used with permission from the American

Chemical Society.

Figure 26.18 Experimental phase diagram for an ABA triblock

copolymer organogel modiﬁ ed with an AB diblock copolymer.

The lines serve as guides for the eye. Adapted from Ref. [29]

and used with permission from John Wiley & Sons, Inc.

26.2 Block Copolymer Gels 813

ity during mixing and evaporate thereafter), at elevated temperatures and then

cooling the solution below the endblock T

to induce glassy crosslinks that serve

to stabilize the gel network. An alternative approach to preparing gels from the

same copolymer and solvent pair is to introduce a solvent directly into the ordered

copolymer by diffusion at temperatures below the endblock T

. As the solvent -

incompatible endblocks – and hence their microdomains – are glassy, they do not

dissolve as the midblocks swell. Midblock swellability depends on both the solubil-

ity of the solvent in the midblock and the extent to which the midblocks stretch

(which is entropically unfavorable), as shown schematically in Figure 26.19 . Gels

produced in this fashion from an ordered copolymer have been referred [99] to as

“ mesogels ” because they retain the characteristics of the neat copolymer mes-

ophase, since fabrication occurs under nonequilibrium conditions. King et al . [100]

have observed that, on progressive swelling, such gels derived from a copolymer

possessing the lamellar morphology retain highly swollen lamellae, as evidenced

by the TEM image shown in Figure 26.20 , even at solvent concentrations that

Figure 26.19 Schematic illustration of the

procedure to generate lamellar mesogels from

midblock - selective solvation of ABA triblock

copolymers. (a) Microphase ordering of the

copolymer from solvent casting or melt

processing; (b) Reduction in temperature so

that the A lamellae are rigid (i.e., glassy or

semicrystalline); (c) Diffusion of solvent into

the B lamellae to induce swelling while

retaining a layered morphology. Reproduced

with permission from Ref. [98] ; © American

Chemical Society.

Figure 26.20 Transmission electron

microscopy image of a triblock copolymer

mesogel demonstrating that the procedure

depicted in Figure 26.19 yields intact styrenic

lamellae (stained) in a solvent - swollen

midblock matrix. Reproduced with permission

from Ref. [100] ; © American Chemical

Society.

814 26 Nanostructured Organogels via Molecular Self-Assembly

would have otherwise induced morphological transformations. Li et al . [101] have

expanded earlier theoretical efforts [98, 99, 102] designed to model mesogels in

terms of swollen brushes by simulating the equilibrium swelling volume fraction

as a function of morphology, bridge fraction and midblock – solvent interaction

parameter. It is interesting to note that the extension and compression behavior

of lamellar mesogels are predicted [102] to be dissimilar. During extension, chain

elasticity dominates, whereas osmotic pressure governs compression. In general,

however, mesogels tend to exhibit improved mechanical properties (expressed in

terms of modulus [100] ) relative to their equilibrium counterparts at the same gel

composition.

26.2.7

Special Cases

26.2.7.1 Liquid Crystals

Generally speaking, liquid crystal s ( LC s) can be envisaged as anisotropic, rod - like

molecules that are capable of developing orientational and/or positional order in

the liquid state [103] . Of the three commonly encountered types of liquid crystal-

line mesophases reported (nematic, smectic and cholesteric, or twisted nematic),

the nematic, wherein the molecules align along a single direction with no posi-

tional order, constitutes the simplest [104] . Thermotropic LCs are temperature -

sensitive, and an increase in temperature causes the nematic mesophase to

disorder into an unstructured, isotropic liquid. Kornﬁ eld and coworkers [105]

have successfully synthesized thermoresponsive triblock copolymer gels contain-

ing a midblock - selective LC solvent. To ensure sufﬁ cient compatibility between

the copolymer and nematic solvent (4 - cyano - 4 - n - pentylbiphenyl; known commer-

cially as 5CB) and to avoid macrophase separation, the copolymer molecule is

designed to have glassy (styrenic) endblocks and a midblock functionalized with

a nematic side group. In this case, the copolymer midblock is soluble in both

the LC and isotropic phases of 5CB. At low copolymer concentrations, the copoly-

mer endblocks are soluble in the isotropic phase, but aggregate in the nematic

phase due to endothermic mixing and a low entropy of mixing. Unlike isotropic

solvents, in which the solvent quality changes gradually with temperature, this

LC solvent undergoes an abrupt change in solvent quality at the relatively sharp

isotropic → nematic phase transition [106] . In essence, the gel exhibits an “ on – off ”

LC response at this transition temperature, thereby imparting the gel with added

functionality. At higher copolymer concentrations (20 wt%), the endblocks also

become insoluble in the isotropic phase of 5CB, in which case the gel network

remains intact even at temperatures above the nematic → isotropic transition.

Mesogels of LC triblock copolymers swollen by a nematic solvent have likewise

been investigated [107] . An interesting ﬁ nding is that the modulus of the gel

can be reversibly changed by applying an electric ﬁ eld. The gel can also be

sheared by applying a ﬁ eld above a certain threshold value, thus evincing

quasi - piezoelectricity.

26.2 Block Copolymer Gels 815

26.2.7.2 Ionic Liquids

Ionic liquid s ( IL s) constitute an emerging class of functional compounds that

exhibit electrical conductivity and possess negligibly low vapor pressure, as well

as broadly tunable physical properties [108] . Lodge and coworkers [26] have fabri-

cated thermoreversible block copolymer gels by dissolving a poly( styrene - b -

ethylene oxide - b - styrene ) ( SEOS ) triblock copolymer in an IL at elevated

temperatures, and allowing the glassy endblocks to self - organize and vitrify upon

cooling. The IL used in this study remains liquid at ambient temperature. It is

interesting to note that the same type of copolymer, with a polar midblock, has

been used [109] to prepare mesogels in conjunction with poly(ethylene glycol)

( PEG ) for enhanced carbon dioxide separation. Characterization of the IL - based

copolymer gel, for which the cgc is 4 wt% at 10 ° C, reveals that the temperature

dependence of the ionic conductivity is comparable to that of the bulk ionic liquid

in the absence of the copolymer network. Similar gels have also been produced

[110] with a poly( N - isopropyl acrylamide - b - ethylene oxide - b - N - isopropyl acryla-

mide) triblock copolymer, which possesses temperature - sensitive endblocks. Gela-

tion in this system can be induced due to the lower critical solution temperature

( LCST ) behavior of the endblock in the IL solvent. Pioneering efforts such as these

are charting the course for future research in the bottom - up design of conductive

gels, especially as the copolymers and solvents can be further modiﬁ ed to improve

both electrical and mechanical properties.

26.2.7.3 Multiblock Copolymers

Thus far, nanostructured organogels composed exclusively of ABA triblock copoly-

mers have been considered. Multiblock copolymers generically designated as

)

copolymers are likewise expected to form stabilizing network structures

in an A - or B - selective solvent. Similar to triblock copolymer gels containing cosur-

factant (diblock copolymer) molecules (cf. Section 26.2.5.2 ), each microdomain in

a multiblock copolymer consists of dangling endblocks, as well as looped and

bridged midblocks, in proportions that depend only on n [111] . While the morpho-

logical and property attributes of solvent - free multiblock copolymers have received

considerable attention [112, 113] , few studies have explored the utility of well -

deﬁ ned multiblock copolymer gels. Bansil and coworkers, for instance, have exam-

ined an ABABA pentablock copolymer in two different solvents: 1,4 - dioxane [114] ,

a slightly good solvent for A and a θ solvent for B; and n - hexane [115] , a strongly

selective solvent for the B blocks. In 1,4 - dioxane solutions, the marginally less -

soluble B blocks appear to be physically connected by swollen A blocks. An inter-

esting result is that a gel network does not develop in these solutions, even at the

highest copolymer concentrations studied, due possibly to (i) an insufﬁ ciently low

fraction of microphase - separated B blocks; or (ii) insufﬁ cient solvent selectivity.

In the presence of n - hexane, macrophase separation occurs at low copolymer

concentrations, whereas gelation is accompanied by solvent expulsion at higher

concentrations. Gindy et al . [116] have attempted to explain this result by perform-

ing Monte Carlo simulations and have proposed that, in dilute solutions, macro-

phase separation occurs when the ratio m / n exceeds a critical value (as observed

816 26 Nanostructured Organogels via Molecular Self-Assembly

experimentally in n - heptane). Gelation at higher copolymer concentrations is

attributed to the association of collapsed insoluble microdomains, similar to mul-

tiplets in ionomers. More complicated gel systems derived from randomly coupled

multiblock copolymers possessing broad block and chain polydispersities (e.g.,

polyurethanes [117] ) have likewise been generated and studied, but their nano-

structures are typically not well - deﬁ ned, which is why they are not considered

further here.

26.2.7.4 Cosolvent Systems

Although ABA triblock copolymer gels normally consist of a single, low - volatility

solvent that is selected to be sufﬁ ciently B - selective and A - incompatible, a mixture

of miscible solvents can certainly be employed to ﬁ ne - tune solvent quality and

controllably alter the phase behavior and physical properties of the resultant gel

[118] . More recent studies [71] have demonstrated that this strategy can likewise

be used to adjust the time - responsive dynamic nature of such gels. The addition

of a triblock copolymer to a saturated tackifying resin, which possesses saturated

ring groups and a T

near or slightly above ambient temperature, yields a time -

dependent viscoelastic system that, upon uniaxial or biaxial deformation, slowly

returns to its original shape. In this case, the elastic restoring force of the copoly-

mer network is thwarted by the high viscosity of the solvent matrix. Results

acquired from dynamic rheology conﬁ rm that both G ′ and G ′ ′ are strong functions

of frequency, indicating that these systems are not gels according to the rheological

criteria listed earlier. The addition of a low - T

aliphatic oil to the system, however,

results in a composition - dependent progressive shift of the frequency spectrum

to higher frequencies. This apparent time - composition equivalence is similar in

effect to time – temperature equivalence [5] and permits the construction of a

superpositioned frequency spectrum over a broader range than could be measured

experimentally (cf. Figure 26.21 ). As frequency relates to reciprocal time, the

behavior of the cosolvent gel at very long or very short times can be accurately

assessed at ambient temperature by simply changing the cosolvent composition.

26.3

Organic Gelator Networks

Organic gelling agents, or gelators , with molecular masses of less than ∼ 2 kg mol

− 1

are referred to as low molar - mass organic gelator s ( LMOG s), and constitute a

growing class of compounds that provides fundamental insight into molecular

self - organization and practical use for applications requiring responsive materials

[1, 119, 120] . In stark contrast to solvated block copolymers that form gels by

microphase separation, LMOGs are generally classiﬁ ed according to their molecu-

lar structure and the intermolecular interactions that promote physical gelation.

In this case, the organogel networks are stabilized via noncovalent physico -

chemical interactions such as hydrogen bonds, π – π stacking, or London dispersion

forces. As with block copolymer gels, gels produced by LMOGs are thermorevers-

26.3 Organic Gelator Networks 817

ible, in which case the load - bearing networks dissolve into the surrounding liquid

matrix upon heating above a composition - dependent dissolution temperature

( T

dis

), but reform upon cooling. Physical gels are typically generated by ﬁ rst heating

a relatively low concentration (typically a few percent by mass) of the LMOG in an

organic solvent or low - T

polymeric liquid until all the components become a

solution, or sol , and then cooling the sol to below the gelation temperature ( T

gel

The value of T

gel

is identiﬁ ed as the temperature at which ﬂ ow is no longer discern-

ible over long periods [119] . It is important to recognize that T

gel

is generally lower

than T

dis

, since more thermal energy is required to break apart and dissolve the

gel network than to form it. Conversely, gel network formation may require a ﬁ nite

degree of supercooling to initiate either (i) crystallization; (ii) precipitation; or (iii)

aggregation of the LMOG, thereby producing a gel [121] .

The resultant gels consist of 3 - D self - assembled ﬁ brillar networks (SAFINs) that

are characterized by entangled nanoscale ﬁ brils exhibiting a high surface - to -

volume ratio. These networks have been visualized by a variety of imaging methods,

including scanning electron microscopy ( SEM ), TEM and atomic force microscopy

( AFM ). Representative examples of SEM [122] and TEM [123] images of SAFINs

are provided in Figures 26.22 and 26.23 , respectively, and demonstrate that the

ﬁ brils can range in size from nanometers to micrometers in width, and from

micrometers to millimeters in length. Due to the large solid – liquid interfacial area,

the matrix solvent is effectively entrapped by capillary forces within the network.

At the macroscopic level, the total volume of solvent is immobilized, resulting in

a solid - like material [124] . Because the networks do not consist of long, elastic

chains (as in block copolymer gels), LMOG - based gels tend to be exquisitely shear -

sensitive, and their networks readily break apart during steady or large - amplitude

Figure 26.21 Frequency spectra shown as

master curves for G ′ and G ′ ′ measured from

triblock copolymer organogels composed of

25 wt% copolymer and a cosolvent (mineral

oil/tackifying resin) that varies in

composition, thereby demonstrating that a

ternary SAMIN system exhibits time –

composition equivalence. The composition -

dependent shift factor ( a

), determined using

a reference composition of 60 wt% tackifying

resin, is included as a function of resin

fraction in the inset.

818 26 Nanostructured Organogels via Molecular Self-Assembly

Figure 26.22 Scanning electron microscopy

images collected at (a) high and (b) reduced

magniﬁ cation from a polycatenar organogel

consisting of 3 wt% gelator, illustrating the

morphology representative of SAFIN

organogels. Reproduced with permission from

Figure 26.23 Transmission electron

microscopy image acquired from a

1 - acetonitrile organogel consisting of

0.07 wt% gelator, conﬁ rming the existence of

nanoﬁ brils (measuring 40 – 70 nm in diameter),

some of which exhibit helical twist with a

pitch of ∼ 150 nm. To improve contrast, the

nanoﬁ brils have been selectively stained.

Adapted from Ref. [123] and used with

permission from John Wiley & Sons, Inc.

oscillatory shear, but reform upon cessation of shear. The kinetics of network

healing depend on the chemistry (and interaction mechanism) of the LMOG, the

concentration of the LMOG, and the quality of the solvent matrix. As gels produced

with LMOGs depend on speciﬁ c intermolecular interactions, this section is divided

into three types of interactions that LMOGs require to promote physical gelation,

namely hydrogen bonding, π – π stacking, and London dispersion forces.

26.3.1

Hydrogen Bonding

Hydrogen bonding is the attractive force that exists between an electronegative

atom and a hydrogen attached to another electronegative atom, thereby imparting

the hydrogen with a partial positive charge. The electronegative atom must possess